Isolating cells expressing secreted proteins

ABSTRACT

A method of detecting and isolating cells that produce a secreted protein of interest (POI) that has a T cell receptor variable domain, comprising: a) constructing a cell line transiently or stably expressing a cell surface capture molecule, which binds the POI, by transfecting the cell line with a nucleic acid that encodes such cell surface capture molecule; b) transfecting said cell simultaneously or subsequently with a second nucleic acid that encodes a POI wherein such POI is secreted; c) detecting the surface-displayed POI by contacting the cells with a detection molecule, which binds the POI; and d) isolating cells based on the detection molecule.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/240,541filed 29 Sep. 2008, which is a continuation-in-part of application Ser.No. 11/434,403 filed 15 May 2006, now U.S. Pat. No. 7,435,553, which isa continuation of U.S. Ser. No. 11/099,158 filed 5 Apr. 2005, nowabandoned, which is a divisional of U.S. Ser. No. 10/050,279 filed 16Jan. 2002, now U.S. Pat. No. 6,919,183, which claims the benefit under35 USC §119(e) of U.S. Provisional Patent Application Ser. No.60/261,999 filed 16 Jan. 2001, which applications are each hereinspecifically incorporated by reference in their entirety.

This application hereby incorporates by reference the Sequence Listingsubmitted in Computer Readable Form as file 790E_SeqList.txt created onJan. 9, 2013 (5047 bytes).

BACKGROUND Field of the Invention

The field of this invention is a method for identifying and isolatingcells that produce secreted proteins. More specifically, the methodallows rapid isolation of high expression recombinant antibody-producingcell lines, or may be applied directly to rapid isolation of specifichybridomas.

Prior art methods for expressing a gene of interest (GOI) in a host cellare known. Briefly, an expression vector carrying the GOI is introducedinto the cell. Following stable integration, standard methods forisolating high expression cells may involve collection of cell pools,hand-picking colonies from plates, isolation of single cells by limiteddilution, or other methods known in the art. Pools or individual clonesare then expanded and screened for production of the protein of interest(POI) by direct measurement of POI activity, by immunological detectionof POI, or by other suitable techniques. These procedures are laborious,inefficient, expensive, and the number of clones that can be analyzed isusually limited to a few hundred.

The large degree of heterogeneity in protein expression by cellsfollowing stable integration requires that many individual clones bescreened in an effort to identify the rare integration event thatresults in a stable, high expression production cell line. Thisrequirement calls for methods that enable rapid identification andisolation of cells expressing the highest level of protein production.Moreover, the collection of clone pools, or hand-picked colonies, riskslosing high expression cells, which often grow more slowly, to fastergrowing low expression cells. Therefore, a need exists for methods thatallow rapid screening and isolation of individual cells capable of highlevel expression of a secreted POI.

Incorporation of flow cytometry into methods used for the isolation ofstable expression cell lines has improved the capability of screeninglarge numbers of individual clones, however, currently available methodsremain inadequate for diverse reasons. Diffusion of the POI betweencells of different characteristics was also a problem.

BRIEF SUMMARY

The present invention describes a high-throughput screening method forthe rapid isolation of those cells that secrete protein by directlyscreening for the protein of interest (POI). This invention also allowsfor the convenient monitoring of POI expression on a single-cell basisduring the manufacturing process. Furthermore, this technology can bedirectly applied to screening of antibody producing cells. Thetechnology can also be directly applied to screening of cells producingmodified T cell receptors, such as, for example, cells that producesoluble forms of T cell receptors.

In one aspect, the invention provides a method of detecting andisolating cells that produce a secreted protein of interest (POI),comprising: a) constructing a nucleic acid molecule that encodes a cellsurface capture molecule capable of binding a POI; b) transfecting acell expressing the POI with the nucleic acid molecule of step a); c)detecting the surface-displayed POI by contacting the cells with adetection molecule, where in the detection molecule binds the POI; andd) isolating cells based on the detection molecule.

In various embodiments, the protein of interest is a ligand, a solublereceptor protein, a growth factor, an antibody, an Fab, a single chainantibody (ScFv), or a fragment thereof. When the protein of interest isan antibody, the antibody is selected from the group consisting of IgM,IgG, IgA, IgD or IgE, as well as various subtypes of these. In aspecific embodiment, the antibody is an anti-DII4 or an anti-IL-6receptor antibody.

In more specific embodiments, the protein of interest is a growth factorselected from the group consisting of Interleukin (IL)-1, IL-2, IL-4,IL-5, IL-6, IL-7, IL-9, IL-10, IL-13, IL-15, IL-16, IL-17, IL-18, IL-21,Ciliary Neurotrophic Factor (CNTF), erythropoietin, Vascular EndothelialGrowth Factor (VEGF), angiopoietin 1 (Ang-1), angiopoietin 2 (Ang-2),TNF, Interferon-gamma, GM-CSF, TGFβ, and TNF Receptor.

In various embodiments, the protein of interest comprises a variabledomain of a T cell receptor. In specific embodiments, the protein ofinterest is a soluble T cell receptor (sTCR), or a protein comprising aT cell receptor extracellular domain fused to an Fc (TCR-Fc), In aspecific embodiment, the Fc is a human Fc. In various embodiments, theprotein comprises a variable domain of a T cell receptor extracellulardomain. In various embodiments, the protein comprises a variable domainand a constant region of a T cell receptor extracellular domain.

The nucleic acid that encodes the protein of interest may be from anysource, naturally occurring or constructed through recombinanttechnology, and may be selected from a DNA library.

In various embodiments, the cell surface capture molecule is aligand-specific receptor, a receptor-specific ligand, anantibody-binding protein, an antibody or antibody fragment, such as anScFv, or a peptide. When the capture molecule is a peptide, the peptidemay be isolated from a phage display library. In more specificembodiments, the capture molecule may be Ang1, And2, VEGF, Tie1, Tie2,VEGFRI (Flt1), VEGFRII (Flk1 or KDR), CNTF, CNTFR-α, cytokine receptorcomponents, fusions of two or more cytokine receptor components, or afragments thereof. When the capture molecule is an antibody-bindingprotein, the antibody-binding protein may be an Fc receptor, ananti-immunoglobulin antibody, an anti-immunoglobulin ScFv, Protein A,Protein L, Protein G, Protein H or functional fragments thereof.

In various embodiments where the protein of interest comprises a T cellreceptor variable domain, the cell surface capture molecule comprises anFc receptor or a membrane-associated antigen capable of being recognizedby the variable domain of the T cell receptor.

In several embodiments, the methods of the invention further comprise amembrane anchor that serves to anchor the POI to the cell membrane,exposed to the outside of the cell, and thus functions as a cell surfacecapture molecule. In specific embodiments, the membrane anchor is atransmembrane anchor or a GPI link. The membrane anchor may be native tothe cell, recombinant, or synthetic.

In various embodiments, the protein of interest comprises a T cellreceptor variable region, and the cell surface capture moleculecomprises a membrane-associated antigen. In a specific embodiment, themembrane-associated antigen is a recombinant fusion protein comprisingan antigen capable of being recognized by the T cell receptor variableregion fused to a membrane anchor wherein the antigen is associated withthe cell surface. In a specific embodiment, the recombinant fusionprotein comprises an antigen fused to a transmembrane anchor or a GPIlink. In another specific embodiment, the cell surface capture moleculecomprises a recombinant fusion protein comprising an membrane anchor andan antigen that is capable of binding to a major histocompatibility(MHC) molecule, including but not limited to, for example, tumorantigens and self proteins of transformed phenotype.

In further embodiments, a signal sequence is added to the amino terminusof a POI, such that the protein is transported to the cell surface, andfunctions as a cell surface capture molecule. The signal sequence may benative to the cell, recombinant, or synthetic.

In various embodiments, a blocking molecule which binds the cell surfacecapture molecule is added to reduce the diffusion of the POI from theexpressing cell to a neighboring cell. In another embodiment, thediffusion of the POI from the expressing cell to a neighboring cell andits adherence to that cell is reduced by increasing the viscosity of themedia.

The cell isolated by the methods of the invention may be anantibody-producing cell fused to an immortalized cell. In more specificembodiments, the antibody-producing cell is a B-cell or derivativethereof. A B-cell derivative may be a plasma cell, a hybridoma, amyeloma, or a recombinant cell.

In addition, the methods of the invention are useful for identificationof B-cells and derivatives thereof, or hybridomas that express secretedantibodies of a desired specificity, affinity or isotype. The inventioncan also be used for isolation of cells that express desired levels ofan antibody or antibody fragments.

Detection of the cells with the displayed POI may be accomplishedthrough the use of any molecule capable of directly or indirectlybinding the displayed POI. Such detection molecules may facilitate thedetection and/or isolation of the cells displaying the POI. In oneembodiment, two molecules that bind each other and are differentiallylabeled are utilized. The detection and/or isolation may be accomplishedthrough standard techniques known in the art.

In another aspect, the invention features a method of detecting andisolating cells that produce a secreted protein of interest (POI),comprising: a) transfecting a cell with a nucleic acid that encodes acell surface capture molecule, wherein the cell surface capture moleculeis capable of binding the POI; b) transfecting the cell of a)simultaneously or subsequently with a second nucleic acid that encodes aPOI wherein the POI is expressed and secreted; c) detecting thesurface-displayed POI by contacting the cell with a detection molecule,which binds the POI; and d) isolating cells based on the detectionmolecule.

In another aspect, the invention features a method of detecting andisolating cells that produce a POI, comprising: a) detecting a cell thatexpresses a cell surface capture molecule in high yield; b) isolatingand culturing the cell detected in (a); c) transfecting the cell in (b)with a nucleic acid that encodes a POI wherein such POI is secreted; d)detecting the surface-displayed POI by contacting the cells with adetection molecule which binds the POI; and e) isolating cells based onthe detection molecule.

In another aspect, the invention provides a method of detecting andisolating cells that produce high levels of protein of interest (POI),comprising: a) transfecting cells with a nucleic acid that encodes suchcell surface capture molecule capable of binding the POI, wherein thecell expresses the POI; b) detecting a cell from (a) that expresses saidcell surface capture molecule in high yield; c) isolating and culturinga high yield cell; d) detecting the surface-displayed POI by contactingthe cell with a detection molecule binds the POI; and e) isolating thedetected cell.

In another aspect, the invention provides a method of detecting andisolating cells that produce high levels of an immunoglobulin,comprising: (a) transfecting cells with a nucleic acid that encodes acell surface capture molecule capable of binding the immunoglobulin,wherein the cell expresses the immunoglobulin; (b) detecting a cell of(a) that expresses the surface capture molecule in high yield; (c)isolating and culturing the cell that expresses the surface capturemolecule in high yield; (d) detecting the immunoglobulin on the surfaceof the isolated and cultured cell of step (c) with a detection moleculethat binds the immunoglobulin; and (e) isolating the cell detected instep (d) that bears the detected immunoglobulin on its surface.

In another aspect, a method for detecting cells that produce a desiredlevel of an affinity agent that comprises a T-cell receptor (TCR)variable region is provided.

In another aspect, a method for detecting cells that produce a desiredlevel of a TCR-Fc is provided, comprising: (a) transfecting cells with anucleic acid that encodes an Fc receptor capable of binding a TCR-Fc,wherein the cell expresses an antigen recognized by the TCR-Fc; (b)detecting a cell of (a) that expresses the TCR-Fc in high yield; (c)isolating and culturing the cell that expresses the TCR-Fc in highyield; (d) detecting the antigen on the surface of the isolated andcultured cell of step (c) with a detection molecule; and (e) isolatingthe cell detected in step (d) that bears the detected antigen on itssurface.

In various embodiments, the TCR is selected from a human TCR and arodent TCR such as a rat, mouse, or hamster TCR. In a specificembodiment the Fc is a human Fc. In another specific embodiment, the Fcis a human Fc and the Fc receptor is a high affinity human Fc receptor.In a specific embodiment, the high affinity human Fc receptor is a humanFcγRI.

In various embodiments, the cell surface capture protein issurface-bound antigen. In a specific embodiment, the antigen is bound tothe surface by fusion to a transmembrane domain or a GPI linker.

Other objects and advantages will become apparent from a review of theensuing detailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents the construction of pTE084, designed for theconstitutive expression of human FcγRI from the upstream CMV-MIEpromoter.

FIG. 2A shows a flow cytometry single parameter histogram of unstainedCHO K1 cells.

FIG. 2B shows a flow cytometry single parameter histogram of FITC-hFcstained CHO K1.

FIG. 2C shows a flow cytometry single parameter histogram of FITC-hFcstained G418-resistant CHO K1 cell pool after pTE084 transfection.

FIG. 2D shows a flow cytometry single parameter histogram of FITC-hFcstained RGC3 cells.

FIG. 3 summarizes the ability of IgG from a variety of animal species toblock 4SC622 from binding to RGC1 cells.

FIG. 4A shows a flow cytometry single parameter histogram of unstainedRGC1 cells.

FIG. 4B shows a flow cytometry single parameter histogram of 4SC622binding to FcγR1-expressing RGC1 cells as indicated by PE-AG184 binding.

FIG. 4C shows a flow cytometry single parameter histogram of rat IgGblocking the binding of 4SC622 to RGC1 cells as indicated by loss ofPE-AG184 binding.

FIG. 5A shows a flow cytometry dual parameter histogram of RGC2 cells,which express the gene for hFcγRI and GFP, stained with PE-AG184.

FIG. 5B shows a flow cytometry dual parameter histogram of RGC2 cells,which express the gene for hFcγRI and GFP, incubated with 1 ug/ml 4SC622for 18 hours before being stained with PE-AG184.

FIG. 5C shows a flow cytometry dual parameter histogram of RGC4 cells,which express the gene for hFcγRI and 4SC622, stained with PE-AG184.

FIG. 5D shows a flow cytometry dual parameter histogram of RGC4 cells,which express the gene for hFcγRI and 4SC622, incubated with rat IgG (1mg/ml) for 18 hours before being stained with PE-AG184.

FIG. 5E shows a flow cytometry dual parameter histogram of a mixture ofRGC2, which express the gene for hFcγRI and GFP, and RGC4 cells, whichexpress the gene for hFcγRI and 4SC622, mixed and incubated together for18 hours prior to staining with PE-AG184.

FIG. 5F shows a flow cytometry dual parameter histogram of a mixture ofRGC2, which express the gene for hFcγRI and GFP, and RGC4 cells, whichexpress the gene for hFcγRI and 4SC622, mixed and incubated together for18 hours with 1 mg/ml rat IgG prior to staining with PE-AG184.

FIG. 6 shows a flow cytometry single parameter histogram of aMSX-resistant pool of RGC1 cells transfected with pEE14.1-622. Cells inthe top 3% gate (R3), 7-11% gate (R5), and 15-19% gate (R7) werecollected, expanded, and their 4SC622 productivity quantitated byimmuno-staining.

FIG. 7 summarizes a comparison of the specific productivities of 4SC622expressing cell lines. CHO K1 transiently transfected with pEE14.1-622,hand-picked stable MSX-resistant clones of CHO K1 transfected withpEE14.1-622, and MSX-resistant 4SC622

production clones isolated after transfection of RGC1 cells withpEE14.1-622.

FIG. 8A shows a flow cytometry single parameter histogram of RGC1 cellsstained with PE-AG184.

FIG. 8B shows a flow cytometry single parameter histogram of RGC1 cellsincubated with 1 ug/ml 4SC622 for 1 hour prior to being stained withPE-AG184.

FIG. 8C shows a flow cytometry single parameter histogram of RGC1 cellsthat were incubated with 1 ug/ml 4SC622 for 1 hour, then incubated inmedium without 4SC622 for 18 hours prior to staining with PE-AG184.

FIG. 9 shows that expression of the gene for hFcγRI results in loss of4SC622 from the culture medium. RGC1 cells, or CHO K1 parental cells,were incubated in medium containing 2 ug/ml 4SC622. The concentration of4SC622 remaining in the media was quantitated by immuno-staining after24 hours, and 72 hours incubation.

FIG. 10 represents the construction of pTE158, designed to allowTetR-regulated expression of human FcγRI. Two repeats of the tetoperator sequence (TetO) are immediately downstream of the CMV promoterin pTE158.

FIG. 11A shows a flow cytometry single parameter histogram of CHO K1cells stained with FITC-hFc.

FIG. 11B shows a flow cytometry single parameter histogram of 10 RGC10cells stained with FITC-hFc.

FIG. 11C shows a flow cytometry single parameter histogram of RGC10cells induced with 1 ug/ml doxycycline for three days prior to stainingwith FITC-hFc.

FIG. 12A shows a flow cytometry single parameter histogram of CHO K1cells stained with polyclonal FITC-conjugated anti-human IgG (H+L)F(ab′)₂ fragment.

FIG. 12B shows a flow cytometry single parameter histogram ofMSX-resistant RGC10 cells transfected with pEE14.1-622 and incubatedwith rat IgG (1 mg/ml) for 18 hours prior to staining with polyclonalFITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

FIG. 12C shows a flow cytometry single parameter histogram ofMSX-resistant RGC10 cells transfected with pEE14.1-622 induced with 1ug/ml doxycycline for three days then incubated with rat IgG (1 mg/ml)for 18 hours prior to staining with polyclonal FITC-conjugatedanti-human IgG (H+L) F(ab′)₂ fragment.

FIG. 13 summarizes the specific productivities of MSX-resistant stableclones of RGC10 cells transfected with pEE14.1-622

FIG. 14 represents the structure of pTE255, designed for theconstitutive expression of human FcγRI from the upstream MoMuSV LTRpromoter.

FIG. 15A shows a flow cytometry single parameter histogram of unstainedSp2/0 cells.

FIG. 15B shows a flow cytometry single parameter histogram of Cy5-hFcstained Sp2/0 cells.

FIG. 15C shows a flow cytometry single parameter histogram of unstainedSp2/0-FcR-4 cells.

FIG. 15D shows a flow cytometry single parameter histogram of Cy5-hFcstained Sp2/0-FcR-4 cells.

FIG. 16 represents the structure of pTE209, designed for theconstitutive expression of 4SC622 from the upstream CMV MIE promoter.

FIG. 17A shows a flow cytometry single parameter histogram of unstainedhygromycin B-resistant Sp2/0-FcR-4 cells transfected with pTE209.

FIG. 17B shows a flow cytometry single parameter histogram of hygromycinB-resistant Sp2/0-FcR-4 cells transfected with pTE209 and incubated withrabbit IgG (1 mg/ml) for 18 hours prior to staining with polyclonalFITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

FIG. 17C shows a flow cytometry single parameter histogram of unstainedcells expanded from the top 1% most fluorescent cells in FIG. 4B.

FIG. 17D shows a flow cytometry single parameter histogram of the cellsexpanded from the top 1% most fluorescent cells in FIG. 4B, incubatedwith rabbit IgG (1 mg/ml) for 18 hours prior to staining with polyclonalFITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

FIG. 17E shows a flow cytometry single parameter histogram of unstainedclone 5H11 cells.

FIG. 17F shows a flow cytometry single parameter histogram of clone 5H11cells incubated with rabbit IgG (1 mg/ml) for 18 hours prior to stainingwith polyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

FIG. 18 shows schematic diagrams of domains of Protein G and ProteinG/hFcγRI fusion protein encoded in pTE300.

FIG. 19 is an outline of the construction of pTE300, designed for theexpression of a chimeric protein containing the RORI signal sequence,the Fc binding domain of Protein G, and the transmembrane andintracellular domain of hFcyRI from the upstream CMV MIE promoter.

FIG. 20A shows a flow cytometry single parameter histogram of unstainedRGC14 cells.

FIG. 20B shows a flow cytometry single parameter histogram of FITC-hFcstained RGC14 cells.

FIG. 20C shows a flow cytometry single parameter histogram of unstainedG418-resistant RGC14 cell pool transfected with pTE300.

FIG. 20D shows a flow cytometry single parameter histogram of FITC-hFcstained G418-resistant RGC14 cell pool transfected with pTE300.

FIG. 20E shows a flow cytometry single parameter histogram of unstainedRGC18 cells.

FIG. 20F shows a flow cytometry single parameter histogram of RGC18cells incubated with 10% fetal bovine serum for 2 hours prior tostaining with polyclonal FITC-conjugated anti-bovine IgG (H+L) F(ab′)₂fragment.

FIG. 21A shows a flow cytometry single parameter histogram of unstainedhygromycin B-resistant cell pool derived from RGC18 after transfectionwith pTE209.

FIG. 21B shows a flow cytometry single parameter histogram of thehygromycin B-resistant cell pool derived from RGC18 after transfectionwith pTE209, incubated with rabbit IgG (1 mg/ml) for 18 hours prior tostaining with polyclonal FITC-conjugated antihuman IgG (H+L) F(ab′)₂fragment.

FIG. 22A shows a flow cytometry single parameter histogram of unstainedRGC18 cells.

FIG. 22B shows a flow cytometry single parameter histogram of RGC18cells incubated with 4SC622 (1 μg/ml) for 1 hour prior to staining withpolyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

FIG. 22C shows a flow cytometry single parameter histogram of RGC18cells incubated with 4SC622 (1 μg/ml) and rabbit IgG (1 mg/ml) for 1hour prior to staining with polyclonal FITC-conjugated anti-human IgG(H+L) F(ab′)₂ fragment.

FIG. 22D shows a flow cytometry single parameter histogram of RGC18cells incubated with 4SC622 (1 μg/ml) for 18 hours prior to stainingwith polyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

FIG. 22E shows a flow cytometry single parameter histogram of RGC18cells incubated with 4SC622 (1 μg/ml) for 2 hours then with 4SC622 (1μg/ml) and rabbit IgG (1 mg/ml) for 18 hours prior to staining withpolyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

FIG. 22F shows a flow cytometry single parameter histogram of unstainedRGC19 cells derived from RGC18 cells by transfection with pTE209.

FIG. 22G shows a flow cytometry single parameter histogram of RGC19cells stained with polyclonal FITC-conjugated anti-human IgG (H+L)F(ab′)₂ fragment.

FIG. 22H shows a flow cytometry single parameter histogram of RGC19cells incubated with rabbit IgG (1 mg/ml) for 18 hours prior to stainingwith polyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

DETAILED DESCRIPTION

Before the present methods are described, it is to be understood thatthis invention is not limited to particular methods, and experimentalconditions described, as such methods and conditions may vary. It isalso to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto be limiting, since the scope of the present invention will be limitedonly by the appended claims.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural references unless the contextclearly dictates otherwise. Thus for example, a reference to “a method”includes one or more methods, and/or steps of the type described hereinand/or which will become apparent to those persons skilled in the artupon reading this disclosure and so forth.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. All publications mentioned herein areincorporated herein by reference to describe in their entirety.

General Description

The method of the invention provides substantial advantages over currentmethods for isolation and identification of protein-secreting cells. Forexample, cells that secrete antibodies may be rapidly and convenientlyisolated based on desired specificity, avidity, or isotype. Furthermore,the amount of secreted protein produced may be directly quantified,unlike many methods in the prior art wherein production of secretedprotein is indirectly quantified.

Recently, two additional methods that utilize flow cytometry have beendeveloped for the high throughput isolation of stable high expressioncell lines. The first method involves modification of the expressionplasmid to include a transcriptional read out for the GOI mRNA. This ismost often accomplished by inserting an internal ribosomal entry site(IRES) and a gene whose protein product is easily monitored by flowcytometry, most frequently green fluorescent protein (GFP), between thestop codon of the GOI and the terminal poly A site (Meng et al. (2000)Gene 242:201). The presence of an IRES allows the POI and GFP to betranslated from the same mRNA. Therefore, the expression level of theGFP gene is indirectly related to the mRNA level for the GOI. Clonesthat accumulate the GFP at high levels are isolated by flow cytometryand then screened for POI production. Because this method depends on thecoupling of GOI expression to the reporter gene by use of an IRES in arecombinant construction, it is not applicable to the isolation ofhybridomas.

The use of flow cytometry in the isolation of expression clones allowsfor the rapid analysis of large numbers of clones in a high throughputformat. Moreover, use of flow cytometry significantly reduces the directhandling of cells. Unfortunately, the level of GFP production is not adirect measure of the production level of the POI. Various mechanismsmay uncouple the production of secreted POI from accumulation of GFP.Differences in production of the POI and the GFP reporter may resultfrom differences in the translation efficiency of the two genes,secretion efficiency of the POI, or stability of the polycistronic mRNA.

Another method that uses flow cytometry to isolate expression clonesinvolves encapsulation of cells within agarose microdrops (Weaver et al.(1990) Methods Enzymol. 2:234). In this method biotinylated antibodiesspecific for the POI are bound to the biotinylated agarose throughstreptavidin such that secreted POI is captured and retained within themicrodrop (Gray et al., (1995) J. Immunol. Methods 182:155). The trappedPOI is detected by immuno-staining with an antibody specific for thePOI. To reduce the encapsulating agarose from absorbing POI secretedfrom adjacent cells, the cells are placed in a low-permeability medium.Those cells with the highest antibody staining of the POI in theembedding agarose are identified and isolated by flow cytometry. The gelmicrodrop approach screens cells directly for their ability to secretePOI, rather than indirectly screening for expression of GOI mRNA, butrequires the availability of suitable antibodies for trapping andstaining the secreted POI and the procedure requires special equipmentto generate the agarose gel microdrops. Moreover, some cells may besensitive to the encapsulation process.

A variation of this method circumvents the requirement for embeddingcells in a matrix by directly binding an antibody, specific for the POI,to the cell surface (Manz et al. (1995) PNAS 92:1921-1925). In thismethod, non-specific biotinylation of cell surface proteins withbiotin-hydroxysuccinimide ester is followed by contact with astreptavidin-conjugated antibody capable of binding the POI. Cellssecreting the POI become decorated with the POI which is then detectedwith an appropriately labeled second antibody. However, diffusion of POIbetween neighboring cells is problematic, and this method also requiresa high viscosity medium to reduce diffusion of POI away from expressingcells. Because these high viscosity media are required fordiscriminating cells, the cells must be washed and placed in a mediumsuitable for cell sorting if so desired.

The problems associated with identification and isolation of highexpression recombinant cell lines especially applies to the isolation ofhybridomas that express an antibody of interest. However, theidentification of useful hybridomas includes several additionalproblems; they must be screened first for antigen-binding activity, thenfor immunoglobulin isotype. Moreover, GFP-based methods are notapplicable to the identification and isolation of hybridomas becauseconstruction of hybridomas does not include a recombinant construct suchthat expression of the antibody genes can be linked to a transcriptionalreporter such as GFP. Hybridoma screening is a slow, laborious endeavorwhere the number of clones screened is limited by existing technologies.

The instant invention describes a novel and previously unknown method ofidentifying and isolating cells that produce secreted proteins. Theinvention is based on the production of a cell line that expresses amolecule, localized to the cell surface, which binds the POI. The cellsurface-displayed POI can then be detected by labeling with variousdetection molecules. The amount of POI displayed on the cell surface,under specific conditions, is a direct measure of the total amount ofPOI secreted. POI producers may then be isolated from non-producers, andlevels of production or POI characteristics may be differentiated. Theadvantage of the invention is that it directly quantifies the secretedPOI rather than indirectly measuring the mRNA.

This invention relates to the construction or use of cells that expresscell surface capture molecules which bind various secreted POIs in thesame cell that produces the POI. As the cell secretes the POI, thesecell surface capture molecules bind it, or complexes of POI and cellsurface capture molecules may form intracellularly and then getsecreted. Binding may occur in an autocrine manner or while beingsecreted. The cells that produce the secreted POI may then be identifiedand isolated. Such identification and isolation may be based oncharacteristics of the POI, production of the POI or lack thereof, or byspecified levels of production. The cell surface capture molecule and/orthe POI may be produced by the cell in its native state, or the cellsurface capture molecules and/or the POI may be recombinantly produced.Through the construction or use of such a cell, any secreted protein maybe captured by the cell surface capture molecule provided there is acorresponding affinity between the two. As explained further, anymolecule may be manipulated such that it can be used as a cell surfacecapture molecule. Therefore, this invention may be utilized to isolateany cell that secretes a protein.

Most any protein has the capacity to function as a cell surface capturemolecule as described by the invention. What is necessary is the abilityof the desired protein to be anchored to the cell membrane and exposedto the extracellular space. If the desired cell has a signal sequencethen only a membrane anchor, including but not limited to atransmembrane anchor or a GPI linkage signal, need be added to the cellsurface capture molecule such that it remains anchored in the cellmembrane exposed to the outside of the cell. Furthermore, if the desiredprotein lacks a signal sequence, a signal sequence may be added to theamino terminus of the desired protein, such that it is transported tothe cell surface. A signal sequence and a membrane anchor may be nativeto the cell, recombinant, or synthetic.

Cells often secrete a wide variety of proteins, endogenously orfollowing the introduction of recombinant DNA. Any secreted protein maybe identified and the cell producing it may be isolated according to themethod of this invention. Such secreted proteins include but are notlimited to growth factors, growth factor receptors, ligands, solublereceptor components, antibodies, sTCRs, TCR-Fc's, and peptide hormones.Such secreted proteins may or may not be recombinant. That is, thesecretion of some proteins of interest from the desired cell may notrequire the introduction of additional nucleotide sequences. Forexample, the secretion of antibodies from B-cells or plasma cells is notthe result of introduction of recombinant nucleotide sequences into theB-cell or plasma cell. Recombinant secreted proteins may be produced bystandard molecular biology techniques well known to the skilled artisan(see e.g., Sambrook, J., E. F. Fritsch And T. Maniatis. MolecularCloning: A Laboratory Manual, Second Edition, Vols 1, 2, and 3, 1989;Current Protocols in Molecular Biology, Eds. Ausubel et al., GreenePubl. Assoc., Wiley Interscience, NY). These secreted proteins areuseful for many commercial and research purposes. This inventionencompasses the production of such secreted proteins through themethodologies of the invention. Detection of the cells with thedisplayed POI may be accomplished through the use of any moleculecapable of directly or indirectly binding the displayed POI. Suchdetection molecules may facilitate the detection and/or isolation of thecells displaying the POI.

The invention is applicable to the isolation of, inter alia, a)ligand-producing cells by using the ligand-specific receptor as the cellsurface capture molecule, b) soluble receptor-producing cells by using asurface bound receptor-specific ligand as the cell surface capturemolecule,—c) antibody-producing cells by using an antibody-bindingprotein as the cell surface capture molecule, d) sTCR's by using ans-TCR-binding protein (e.g., and antigen recognized by the TCR) as thecell surface capture molecule, or e) TCR-Fc's, by using an Fc-bindingprotein as a cell surface capture molecule.

In accordance with the methodology of this invention, a cell is firsttransfected with a vector containing a nucleotide sequence that encodesa cell surface capture molecule that is capable of binding the secretedPOI, under conditions in which such cell surface capture molecule isexpressed. Transfected cells which are appropriate producers of suchcell surface capture molecules are then detected and isolated, and suchcells are cultured. These cells may either naturally produce the POI, orthe POI may be recombinantly produced. If the cells naturally producethe POI, they are ready for detection and isolation. If the POI is to berecombinantly produced, then the isolated and cultured cells expressingthe specified cell surface capture molecule are transfected with secondnucleotide sequence that encodes the secreted POI, under conditions inwhich the secreted POI is expressed. Upon expression, the secreted POIbinds to the cell surface capture molecules and the cells displayingbound POI are detected and isolated.

If the POI is naturally produced by the cell, the cell will not betransfected with nucleotide sequence encoding the POI. Therefore, thisaspect of the invention is applicable to any and all cells producing aPOI. In addition, if the cell surface capture molecule is naturallyproduced by the cell, the cell need not be transfected with nucleotidesequences encoding the cell surface capture molecule. Therefore, thisaspect of the invention is applicable to any and all cells producing acell surface capture molecule.

A wide variety of host cells may be transfected. These cells may beeither of eukaryotic or of prokaryotic origin. The cells will often beimmortalized eukaryotic cells, and in particular, mammalian cells, forexample monkey kidney cells (COS), Chinese hamster ovary cells (CHO),HeLa cells, baby hamster kidney cells (BHK), human embryonic kidneycells (HEK293), leukocytes, myelomas, cell lines transfected withadenovirus genes, for example, AD5 E1, including but not limited toimmortalized human retinal cells transfected with an adenovirus gene,for example, PER.C6™ cells, and embryonic stem cells. The cells may alsobe non mammalian cells including bacterial, fungi, yeast and insectcells, including, but not limited to, for example Escherichia coli,Bacillus subtilus, Aspergillus species, Saccharomyces cerevisiae, andPichia pastoris. All cells may be grown in culture trays medium underappropriate conditions or in a synergistic host. The most desirablecells will be mammalian cells capable of culture.

The secreted POI bound to the cell surface capture molecule may bedetected and isolated by various techniques known in the art. Culturescells displaying the secreted POI may be contacted with (a) molecule(s)capable of directly or indirectly binding the secreted POI wherein suchdetection molecule(s) may contain a detection label, such as, forexample, a chromogenic, fluorogenic, colored, fluorescent, or magneticlabel. The label bound to the detection molecule may be detected and thecell isolated using various methods. Most preferably, within a cellpopulation the label will be detected and the cell isolated utilizingflow cytometry. Alternatively, the detection molecule may be used forthe direct isolation of cells displaying the POI. This may beaccomplished by conjugation of the detection molecule to a cultureplate, paramagnetic molecules, or any other particle or solid support.In addition, displayed POI may be detected directly by a property of thedetection molecule or the POI.

In one embodiment, two detection molecules that bind each other and aredifferentially labeled are used to detect a displayed secreted POI thatblocks that interaction. If a cell displays a secreted POI that bindsthe first detection molecule and blocks the interaction between thefirst and second detection molecule, that cell may be isolated based onthe presence of only the first detection molecule on its surface. On theother hand, if a cell displays a secreted POI that binds the firstdetection molecule but does not block the interaction between the firstand second detection molecule, that cell may be isolated based on thepresence of both detection molecules on its surface. For example,antibody producing cells expressing antibodies that specifically block,or do not block, the formation of a receptor-ligand complex may beidentified. If the detection molecules are a receptor and its ligandwhich are differentially labeled, then an antibody producing cell thatexpresses antibodies that block the receptor-ligand complex from formingmay be detected by the presence of one label on its surface, whereas anantibody producing cell that expresses antibodies that do not block thereceptor-ligand complex from forming may be detected by the presence ofboth labels on its surface.

In any of the embodiments and with regards to isolating expressing cellsfrom non-expressing cells or lesser expressing cells, one of theprincipal difficulties, when the POI is a secreted protein, is diffusionof POI between neighboring cells. Therefore, it is critical that anysystem that is designed to capture the secreted POI on the cell surfacemust prevent the diffusion of the POI from the expressing cell to aneighboring cell and its adherence to that cell. If diffusion is allowedto occur, and neighboring cells become decorated with the secreted POI,then separation of cells based upon the degree of POI decoration willfail to discriminate high expressing cells from cells with lowexpression levels, and may fail to effectively isolate expressing fromnon-expressing cells.

Therefore one embodiment of this invention is to block the diffusion ofthe secreted POI between neighboring cells. This may be accomplished bythe addition of a blocking molecule that binds either the cell surfacecapture molecule or the POI and prevents the binding of the secreted POIto the cell surface capture molecule. In this aspect, the detectionmolecules do not bind the blocking molecule. For example, if the cellsurface receptor is the hFcγRI and the secreted POI possesses the humanIgG Fc fragment, then diffusion of the secreted POI between neighboringcells may be blocked by the addition of exogenous rat IgG to the culturemedia. Detection of cells displaying secreted POI, and not bound ratIgG, is achieved by use of antibodies specific for human IgG Fc that donot recognize rat IgG. In another embodiment, binding of the secretedPOI between neighboring cells is reduced by increasing the viscosity ofthe media.

In one embodiment of this invention, the secreted POI is not allowed toaccumulate in the media. This may be accomplished by regulating theexpression of the secreted POI and/or the cell surface capture moleculesuch that brief expression of the POI results in sufficient POI to bindthe cell surface capture molecule but insufficient amounts fordiffusion. In another embodiment, cells may be removed from the mediacontaining accumulated POI, the POI bound to the cells is stripped off,and POI expression is allowed to continue for a limited period of timesuch that secreted POI does not accumulate in the media. Proteins may bestripped by methods known in the art, for example, washing cells withlow pH buffer.

According to this invention, those cells in a cell population that bindthe most detection molecules also express the most secreted POI. Infact, the more POI that an individual cell secretes, the more POI isdisplayed on the cell surface. This correlation between the amount ofsurface-displayed POI and the expression level of the POI in that cellallows one to rapidly identify cells with a desired relative expressionlevel from a population of cells.

In one embodiment, a DNA library may be used to express secreted proteinwhich may be displayed on the cell surface by the cell surface capturemolecule. For example, a library of DNA may also be generated from thecoding regions of the antibody variable domains from B-cells isolatedfrom immunized animals. The DNA library may then be expressed in a cellthat expresses a cell surface capture molecule specific for antibodiessuch that clones of desired specificity, isotype, or avidity may beidentified and isolated by the method of the invention. In anotherembodiment, a library of DNA may be generated from the coding regions ofT cell receptor variable domains from T-cells, and fused to, forexample, an Fc capable of binding to an Fc-binding protein. The DNAlibrary may them be expressed in a cell that expresses an Fc-bindingprotein such that clones of desired specificity, isotype, or avidity maybe identified and isolated as described herein.

In another embodiment, transgenic mammals may be created that express aparticular cell surface capture molecule in one or more cell types. Thecells from such transgenic mammals may then be screened directly for theproduction of a POI. For example, it may be desirable to express a cellsurface capture molecule, specific for antibodies, in plasma cells.Accordingly, plasma cells from immunized mice may be harvested and thosecells producing antibodies specific to the desired antigen may beisolated by the method of the invention.

In a further embodiment of the invention, antibody production ismeasured through the use of a CHO cell line that expresses the humanFcγR1 receptor (FcγRI) which binds the particular antibody or TCR-Fcthat is the POI.

In another aspect of the invention, the protein of interest comprisesone or more T cell receptor variable domains or a soluble T cellreceptor. The one or more T cell receptor variable domains can becovalently linked to a moiety that can bind a cell surface captureprotein. In a specific embodiment, the one or more T cell receptorvariable domains are fused to an Fc sequence, e.g., a human Fc sequence,and the cell surface capture protein is an Fc receptor, e.g., an FcγR.

The general structures of TCR variable domains are known (see, e.g.,Lefranc and Lefranc (2001) The T Cell Receptor FactsBook, AcademicPress, incorporated herein by reference; see, e.g., pp. 17-20; see also,Lefranc et al. (2003) IMGT unique numbering for immunoglobulin and Tcell receptor variable domains and Ig superfamily V-like domains,Developmental and Comparative Immunology 27:55-77, and Lefranc et al.(2005) IMGT unique numbering for immunoglobulin and T cell receptorconstant domains and Ig superfamily C-like domains, Developmental andComparative Immunology 29:185-203, each incorporated herein byreference). In one embodiment, a TCR variable domain of a TCR-Fccomprises an N-terminal region having a variable domain of 104-125 aminoacids. In another embodiment, the TCR-Fc further comprises a TCRconstant region comprising 91-129 amino acids. In another embodiment,the TCR-Fc further comprises a connecting peptide comprising 21-62 aminoacids.

In one embodiment, the Fc sequence is fused directly or through a linkerto the TCR variable domain. In another embodiment, the TCR-Fc comprisesa TCR variable region and a TCR constant region, and the Fc sequence isfused directly or through a linker to the TCR constant region. Inanother embodiment, the TCR-Fc comprises a TCR variable region, a TCRconstant region, and a connecting peptide, and the Fc sequence is fuseddirectly or through a linker to the connecting peptide.

The sTCR, TCR-Fc, or fusion protein comprising one or more T cellreceptor variable regions can be selected so as to specifically bind anantigen of interest, for example, a substance produced by a tumor cell,for example, tumor cell substance that is capable of producing an immuneresponse in a host. In a specific embodiment, the antigen is an antigenthat is present on the surface of a tumor cell (i.e., a tumor antigen),is recognized by a T cell, and that produces an immune response in ahost. Tumor antigens include, for example, alphafetoprotein (AFP),carcinoembryonic antigen (CEA), MUC-1, epithelial tumor antigen (ETA),tyrosinase (e.g., for malignant melanoma), melanoma-associated antigen(MAGE), and mutated or abnormal forms of other proteins such as, forexample, ras, p53, etc.

In one embodiment, the POI is a TCR-Fc, and the TCR-Fc comprises a TCR αchain variable region fused to an Fc sequence and a TCR β chain fused tothe Fc sequence (each directly or through a linker), wherein the TCR αchain-Fc fusion and the TCR β chain-Fc fusion associate to form an αβTCR-Fc. In a specific embodiment, the αβ TCR-Fc comprises the followingtwo polypeptides: (1) a TCR α chain variable region fused to a TCR αchain constant region fused to an Fc sequence, and (2) a TCR β chainvariable region fused to a TCR β chain constant region fused to an Fcsequence.

In another embodiment, the POI is a TCR-Fc having a TCR α variableregion and a TCR β variable region and, optionally, a TCR α constantregion and/or a TCR β constant region. In a specific embodiment, theTCR-Fc is encoded by a nucleic acid comprising (5′ to 3′) a TCR αvariable region sequence, optionally followed by a TCR α constant regionsequence, a TCR β variable region sequence, optionally followed by a TCRβ constant region sequence, optionally a linker, then an Fc sequence. Ina specific embodiment, the TCR-Fc is encoded by a nucleic acidcomprising (5′ to 3′) a TCR β variable region sequence, optionallyfollowed by a TCR β constant region sequence, a TCR α variable regionsequence, optionally followed by a TCR α constant region sequence,optionally a linker, then an Fc sequence. In various embodiments,constructs encoding TCR-Fc's are preceded by signal sequences, e.g.,secretion signal sequences, to render them secretable.

In another embodiment, the POI is a TCR-Fc, and the TCR-Fc comprises aTCR-Fc comprising a TCR γ chain fused to an Fc sequence and a TCR δchain variable region fused to an Fc sequence to form a γδ TCR-Fc. In aspecific embodiment, the γδ TCR-Fc comprises the following twopolypeptides: a TCR a chain variable region fused to a TCR γ chainconstant region fused to an Fc sequence, and (2) a TCR δ chain variableregion fused to a TCR δ chain constant region fused to an Fc sequence.

T cell receptor variable regions can be identified and/or cloned by anymethod known in the art. The T cell receptor variable regions of theprotein of interest are obtainable, for example, by expressingrearranged T cell receptor variable region DNA in a cell, for example,fused to a human Fc sequence. Rearranged T cell receptor variableregions specific for a particular antigen can be obtained by anysuitable method known in the art (see references below), for example, byexposing a mouse to an antigen and isolating T cells of the mouse,making hybridomas of the T cells of the mouse, and screening thehybridomas with the antigen of interest to obtain a hybridoma ofinterest. Rearranged T cell variable regions specific for the antigen ofinterest can be cloned from the hybridoma(s) of interest. T cellreceptor variable regions specific for an antigen can also be identifiedusing phage display technology, for example, as provided in referencesbelow. The variable regions can then be cloned and fused, for example,to a human Fc to make a protein of interest that can bind to a cellsurface capture molecule that is an FcγR.

Methods for identifying and/or cloning T cell receptor variable regionsare described, for example, in U.S. Pat. No. 5,635,354 (primers andcloning methods); Genevée et al. (1992) An experimentally validatedpanel of subfamily-specific oligonucleotide primers (Vα1-w29/Vβ1-w24)for the study of human T cell receptor variable V gene segment usage bypolymerase chain reaction, Eur. J. Immunol. 22:1261-1269 (primers andcloning methods); Gorski et al. (1994) Circulating T Cell RepertoireComplexity in Normal Individuals and Bone Marrow Recipients Analyzed byCDR3 Size Spectratyping, J. Immunol. 152:5109-5119 (primers and cloningmethods); Johnston, S. et al. (1995) A novel method for sequencingmembers of multi-gene families, Nucleic Acids Res. 23/15:3074-3075(primers and cloning methods); Pannetier et al. (1995) T-cell repertoirediversity and clonal expansions in normal and clinical samples,Immunology Today 16/4:176-181 (cloning methods); Hinz, T. and Kabelitz,D. (2000) Identification of the T-cell receptor alpha variable (TRAV)gene(s) in T-cell malignancies, J. Immunol. Methods 246:145-148 (cloningmethods); van Dongen et al. (2002) Design and standardization of PCRprimers and protocols for detection of clonal immunoglobulin and T-cellreceptor gene recombinations in suspect lymphoproliferations: U.S. Pat.No. 6,623,957 (cloning methods and primers); Report of the BIOMED-2Concerted Action BMH4-CT98-3936, Leukemia 17:2257-2317 (primers andcloning methods); Hodges et al. (2002) Diagnostic role of tests for Tcell receptor (TCR) genes, J. Clin. Pathol. 56:1-11 (cloning methods);Moysey, R. et al. (2004) Amplification and one-step expression cloningof human T cell receptor genes, Anal. Biochem. 326:284-286 (cloningmethods); Fernandes et al. (2005) Simplified Fluorescent Multiplex PCRMethod for Evaluation of the T-Cell Receptor Vβ-Chain Repertoire, Clin.Diag. Lab. Immunol. 12/4:477-483 (primers and cloning methods); Li, Y.et al. (2005) Directed evolution of human T-cell receptors withpicomolar affinities by phage display, Nature Biotech. 23/3:349-354(primers and cloning methods); Wlodarski et al. (2005) Pathologic clonalcytotoxic T-cell responses: nonrandom nature of the T-cell receptorrestriction in large granular lymphocyte leukemia, Blood 106/8:2769-2780(cloning methods); Wlodarski et al. (2006) Molecular strategies fordetection and quantitation of clonal cytotoxic T-cell responses inaplastic anemia and myelodysplastic syndrome, Blood 108/8:2632-2641(primers and cloning methods); Boria et al. (2008) Primer sets forcloning the human repertoire of T cell Receptor Variable regions, BMCImmunology 9:50 (primers and cloning methods); Richman, S, and Kranz, D.(2007) Display, engineering, and applications of antigen-specific T cellreceptors, Biomolecular Engineering 24:361-373 (cloning methods).Examples of sTCRs are provided in, for example, U.S. Pat. Nos. 6,080,840and 7,329,731; and, Laugel, B et al. (2005) Design of SolubleRecombinant T Cell Receptors for Antigen Targeting and T CellInhibition, J. Biol. Chem. 280:1882-1892; incorporated herein byreference. Fc sequences are disclosed herein; examples of Fc sequences,and their use in fusion proteins, are provided, for example, in U.S.Pat. No. 6,927,044 to Stahl et al. All of the foregoing references areincorporated herein by reference.

EXAMPLES

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the methods and compositions of the invention, and are notintended to limit the scope of what the inventors regard as theirinvention. Efforts have been made to ensure accuracy with respect tonumbers used (e.g., amounts, temperature, etc.) but some experimentalerrors and deviations should be accounted for. Unless indicatedotherwise, parts are parts by weight, molecular weight is averagemolecular weight, temperature is in degrees Centigrade, and pressure isat or near atmospheric.

Example 1

Construction of pTE084.

pTE084 was constructed by ligating the 1,436 bp Xba I fragment frompCAE100 that encodes the human FcγRI (hFcγRI; GenBank accession numberM21091) into the Xba I site of pRG821. The orientation of hFcγRI indesirable plasmids resulting from the ligation was examined byrestriction mapping with Not I, Pst I, Eco RI, and Stu I. pTE084 wasdesigned for the high level expression of hFcγRI, the high affinity cellsurface receptor for the Fc domain of human IgG. It contains twoindependent expression cassettes. One cassette is a hFcγRI gene drivenby the CMV-MIE promoter, and the second cassette is the neomycinphosphotransferase II (npt) gene, which confers resistance to G418,driven by the SV40 late promoter.

Construction of a CHO K1 Derivative that Expresses hFcγRI.

CHO K1 cells (4×10⁶) were transfected with pTE084 using Lipofectamine™(Life Technologies; Rockville, Md.) following manufacturer'ssuggestions. The cells were placed in the culture medium (10% fetalbovine serum, 90% Ham's F-12, 2 mM L-glutamine; all reagents were fromLife Technologies, Rockville, Md.) containing 500 μg/ml G418 (LifeTechnologies) for 15 days. The cells that survived G418 selection weretrypsinized, pooled, and stained with FITC-conjugated human IgG, Fcfragment (FITC-hFc; Jackson ImmunoResearch Laboratories, West Grove,Pa.). Briefly, the cells grown on 10 cm culture plates were washed oncewith Dulbecco's phosphate-buffered saline (PBS) without calcium chlorideand magnesium chloride (Life Technologies). Three mls of 0.25% trypsin(Life Technologies) was added to each plate. The plates were swirleduntil the cells detached from the plate. Ten ml culture medium wasimmediately added to each plate of the detached cells. The cells werethen collected by centrifugation at 1,000×g for 4 minutes. After removalof supernatant, the cells were resuspended in 4 ml of 2 μg/ml FITC-hFcdiluted in culture medium. The cells were then placed on a platformshaker and stained for one hour at room temperature. To remove unboundFITC-hFc, the cells were washed twice with 20 ml PBS. The degree ofFITC-hFc label on the cells was measured by flow cytometry on a MOFLO™cell sorter (Cytomation; Fort Collins, Colo.). The FITC-hFc did notstain mock-transfected parental CHO K1 cells but gave rise to adistribution of fluorescence in the G418-resistant, pTE084-transfectedpool. The top 1% most fluorescent cells from the selected pool wereplaced into 96-well plates at 1 cell/well by flow cytometry. Nine dayslater, 88 cell clones in the 96-well plates were expanded into 24-wellplates. After 3 days, the cells in individual wells were washed oncewith 1 ml PBS, stained with 0.5 ml of 2 μg/ml FITC-hFc for 1 hour,washed twice with 1 ml PBS and examined for cell surface staining undera fluorescent microscope. The thirty three most fluorescent clones werechosen, expanded, then screened by flow cytometry.

Diffusion of secreted protein between expressing cells andnon-expressing cells among cells was blocked by adding IgG: As all cellsin a hFcγRI clonal cell line express a cell surface hFcγRI, they allpossess the ability to bind IgG or fusion proteins consisting of the Fcdomain of IgG. Because hFcγRI binds IgG from a variety of species (vande Winkel and Anderson, 1991), a panel of animal IgGs was tested for theability to block the binding of a protein containing a human IgG1(hIgG1) Fc tag (4SC622) to hFcγRI-expressing cells. 4SC622 is a chimericmolecule consisting of IL-2Rγ extracellular domain fused to the hIL-4Rγextracellular domain which is then fused to the hIG-1Fc domain. In thisexperiment, cultures of RGC1, a hFcγRI-expressing cell line selectedfrom CHO K1 cells that have been stably transfected with pTE084, wereincubated with 1 μg/ml 4SC622 for 18 hours in the presence or absence of1 mg/ml IgG from different species in a 37° C. tissue culture incubator.

Cell surface binding of 4SC622 was determined by flow cytometry afterwashed cells were stained with phycoerythrin-conjugated mouse IgG1monoclonal AG184 (PE-AG184) specific for the hIL-2Rγ component of 4SC622(BD Pharmingen; San Diego, Calif.), following procedures outlined forcell staining with FITC-hFc.

It was found that hIgG completely blocked 4SC622 from binding to thehFcγR1 expressed on the surface of RGC1. Rat, rabbit and canine-derivedIgG also effectively blocked binding whereas bovine and ovine-derivedIgG did not block. The ability of exogenously added rat IgG to block thebinding of an exogenously added hIgG1 Fc-tagged protein (4SC622) to cellsurface hFcγRI suggests that rat IgG can also block transfer betweencells expressing a hIgG1 Fc-tagged protein at different levels. To testthis, two cell lines that can be distinguished by the presence orabsence of the green fluorescent protein (EGFP) were generated fromRGC1. Briefly, to mark RGC1 cells with EGFP, 2×10⁶ RGC1 cells wereco-transfected with 0.5 mg PTE073 which encodes a hygromycin Bphosphotransferase gene driven by phosphoglycerate kinase promoter, and5 mg pRG816-EGFP which encodes EGFP gene driven by CMV-MIE promoter. Thetransfected cells were selected with 200 μg/ml hygromycin B (Sigma; St.Louis, Mo.) for two weeks. Green fluorescent cells were isolated by flowcytometry. One EGFP and hFcγRI-expressing clone, RGC2, was used in cellmixing experiments. The other cell line used in these experiments, RGC4,was generated by stable transfection of RGC1 with plasmid pEE14.1-622.pEE14.1-622 is a plasmid in which expression of 4SC622 is driven by theCMV-MIE promoter and includes a glutamine synthetase minigene, whichconfers resistance to the analog methionine sulfoximine (MSX), andallows for selection of stable integration events. RGC4 cells expresshFcγRI on the cell surface and secrete the hIgG1 Fc-tagged protein4SC622. One plate of mixed cells comprising 50% RGC2 and 50% RGC4 cellswas incubated with 1 mg/ml rat IgG for 18 hours prior to staining withPE-AG184 then examined by flow cytometry. EGFP fluorescence of RGC2cells shows that RGC2 cells also bind exogenously added 4SC622 (1 μg/ml)as indicated by an increase in PE-AG184 fluorescence. RGC4 did notfluoresce in the EGFP gate. Significantly, exogenously added rat IgG didnot reduce the percentage of RGC4 cells that stained positive for cellsurface 4SC622, suggesting that the binding of 4SC622 to hFcγRI occurredwhile the proteins were in transit to the cell surface. When RGC2 andRGC4 cells were mixed, the 4SC622 protein secreted from RGC4 cellsaccumulated in the medium and bound most of the RGC2 cells. However, theaddition of 1 mg/ml rat IgG significantly reduced the percentage of RGC2cells that bound 4SC622, demonstrating that rat IgG blocked the transferof secreted hIgG1 Fc-tagged protein from expressing cells tonon-expressing cells.

Example 2 Cell Surface Fluorescence Correlates with the Expression Levelof 4SC622

RGC1 cells (4×10⁶) were transfected with pEE14.1-622 and a pool ofstable transfectants was obtained after selection for 2 weeks in mediumcomprised of 10% dialyzed fetal bovine serum, 90% glutamine-freeDulbecco's Modified Eagle's Medium (DMEM), 1×GS supplement, and 25 μMMSX (All reagents were from JRH Biosciences, Lenexa, Kans.). Rat IgG wasadded to the culture medium to 1 mg/ml 18 hours prior to immunostaining.The cells were trypsinized, washed with PBS, and stained with 1.5 μg/mlof a polyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment(Jackson ImmunoResearch Laboratories) for one hour at room temperaturefollowing procedures as described for FITC-hFc staining in Example 1.Cell staining was then analyzed by flow cytometry. The distribution offluorescence suggested that the selected pool contained cells with awide range of 4SC622 expression levels. Cells in the top 3% (R3bracket), 7-11% (R5 bracket), and 15-19% (R7 bracket) with respect totheir immunofluorescence were sorted into three distinct pools andexpanded for 9 days. Average 4SC622 production per cell for the poolswas determined by measuring cell numbers and 4SC622 levels in the mediaafter 3 days growth by an immuno-based Pandex assay (Idexx; Westbrook,Me.) following the manufacturer's recommendations. In the Pandex assay,fluoricon polystyrene assay particles coated with goat anti-human IgG,g-chain specific antibody (Sigma) were used to capture 4SC622 from themedium, and a FITC-conjugated goat anti-human IgG, Fc specific (Sigma)was used to detect bead-bound 4SC622. Known amounts of purified 4SC622were included in the assay for calibration. Cells in the top 3%, 7-11%,and 15-19% pool were found to produce 4SC622 at 1.42, 0.36, and 0.22pg/cell/day, respectively. Thus, there was a correlation between cellsurface 4SC622 staining and specific protein production. This resultsuggests that individual cells that express 4SC622 at high levels may beobtained by isolating cells that were stained brightest by thepolyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.

Example 3 Isolation of Expression Clones in RGC1: IL-4 Trap

To directly demonstrate the efficiency in generating clonal cell lineswith high level secreted protein production by our methodology, clonal4SC622 producing cell lines were generated from RGC1. RGC1 cells (4×10⁶)were transfected with pEE14.1-622, and selected for two weeks with 25 μMMSX to obtain a pool of stable transfectants. MSX-resistant cells werepooled and incubated with 1 mg/ml human IgG for 18 hours, prior tostaining with PE-AG184. Six cells from the top 5% gate, as determined byflow cytometry analysis of cell surface 4SC622 staining, were isolatedand expanded. 4SC622 production from the six clonal lines was determinedand compared to 4SC622 production from clones obtained by hand-pickingselected colonies followed by dilution cloning and amplification. OneRGC1-derived clone, RGC4, produced 4SC622 at 12 pg/cell/day. This levelis similar to that of the best 4SC622 producer isolated by hand-pickingand analyzing 2,700 clones. Thus, compared with hand-picking colonies,the methodology outlined in this invention proves to be far moreefficient in the screening and cloning of high producers.

VEGF Trap.

Plasmids pTE080 and pTE081 encode the genes for VEGF Traps, hVEGFR1R2and hVEGF-R1R3. hVEGF-R1R2 is a chimeric molecule consisting of thefirst Ig domain of hVEGFR1 fused to the second Ig domain of hVEGFR2which is then fused to the hIg1 FC domain. hVEGFR1R3 is a chimericmolecule consisting of the first Ig domain of hVEGFR1 fused to thesecond Ig domain of hVEGFR3 which is then fused to the hIg1 FC domain.In these plasmids, the gene for the VEGF Trap is driven by the CMV-MIEpromoter and a glutamine synthetase minigene, which confers resistanceto MSX, is expressed for selection of stable integration events. RGC1cells were transfected with either of these plasmids and grown in mediumcontaining 25 μM MSX for 2 weeks to select for cells in which theplasmid has stably integrated. MSX-resistant cells were incubated with0.1 μg/ml Ig2a and mouse IgG3 for 18 hours prior to staining with 1.5μg/ml polyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment.Cell were stained for 1 hour then washed twice with PBS prior to flowcytometry. Single cells were sorted into 96-well tissue culture platesfrom the pool of cells whose fluorescence was among the highest 1%. Thecells in individual wells were expanded and their productivities weredetermined by Pandex assays. RGC-derived clones expressing bothhVEGFR1R2 and hVEGFR1R3 had higher specific productivities and wereisolated by screening fewer clones as compared to the highest-expressinghand-picked MSX-resistant colonies. See Table 1.

TABLE I SPECIFIC PRODUCTIVITY COMPARISON Hand-picked CHO K1 RGC1-derivedTran- Stable Cell Lines Stable Cell Lines sient Sp. Prod. # clones Sp.Prod. # clones Protein (μg/ml) (pg/cell/day) screened (pg/cell/day)screened 4SC622 1.1 12 2700 12 6 hVEGF- 33 68 190 77 62 R1R2 hVEGF- 27 5100 22.6 42 R1R3

Example 4 Cell Surface-Bound hIgG1 Fc-Tagged Protein is Internalized byRGC1

hFcγRI is known to induce internalization of its cell surface-boundligand. To analyze whether RGC1 cells could internalize cellsurface-bound 4SC622, 1 μg/ml 4SC622 was added to RGC1 cells for 1 hourand then the cells were immediately processed for 4SC622 immunostainingwith PE-AG184 and flow cytometry analysis. Ninety-three percent of thecells stained positive for cell surface 4SC622. Alternatively, 1 μg/ml4SC622 was added to RGC1 cells for 1 hour, then the cells were washedand incubated in culture medium without 4SC622 with PE-AG184 for 18hours. Flow cytometry analysis following immunostaining for 4SC622showed that 9% of the cells retained 4SC622 on the cell surface. Tofurther characterize the loss of surface-bound 4SC622, purified 4SC622protein was added to the media of RGC1 and parental CHO K1 cells, thenlevels of 4SC622 in the media were measured over time. 4SC622, added to2 μg/ml to the culture media in a 10 cm plate, was significantly lowerin RGC1 conditioned medium after 3 days incubation as compared to theCHO K1 control. These results show that the concentration of 4SC622 inthe culture medium is reduced by the presence of hFcγRI on the cellsurface. The results suggest that the depletion of 4SC622 from the mediawas the result of hFcγRI-4SC622 complex internalization. Thisinternalization of receptor-ligand complexes may facilitate theeffective removal of all 4SC622 from non-expressing cells in thepresence of blocking IgG during the 18-hour blocking step.

Example 5 Construction of CHO K1 Cell Lines with Inducible hFcγRIExpression

Flow cytometry-based autologous secretion trap (FASTR) methods thatutilize the hFcγRI allow rapid isolation of high expression clones.However, if hFcγRI mediates turnover of Fc-tagged proteins, then therealized production of the secreted protein by engineered hFcγRIexpressing cells would be higher if hFcγRI expression could be inhibitedduring the production period. To this end, a CHO K1 cell line in whichthe expression of hFcγRI is induced by tetracycline, or the analogdoxycycline, was constructed. In this system, CHO K1 cells were firstengineered to express the tetracycline repressor protein (TetR) andhFcγRI was placed under transcriptional control of a promoter whoseactivity was regulated by TetR. Two tandem TetR operators (TetO) wereplaced immediately downstream of the CMV-MIE promoter/enhancer in pTE084to generate pTE158-. Transcription of hFcγRI from the CMV-MIE promoterin pTE158 was blocked by TetR in the absence of tetracycline or someother suitable inducer. In the presence of inducer TetR protein wasincapable of binding TetO and transcription of hFcγRI occurs.

CHO K1 cells were transfected with pcDNA6/TR, a plasmid that confersresistance to blasticidin in which expression of TetR originates fromthe CMV-MIE promoter (Invitrogen; Carlsbad, Calif.). After two weeks ofselection with 2.5 μg/ml blasticidin (Invitrogen), the stabletransfectants were pooled. This pool was then transfected with pTE158, aplasmid that confers resistance to G418 in which the expression ofhFcγRI is dependent on a CMV-MIE/TetO hybrid promoter. The cellsconsecutively transfected with pcDNA6/TR and pTE158 were selected with400 μg/ml G418 and 2.5 μg/ml blasticidin for 12 days then pooled. Thepool was induced for two days by the addition of 1 μg/ml doxycyclinethen stained with FITC-hFc to identify cells that express hFcγRI. Thetop 5% of cells expressing hFcγRI were collected as a pool, expanded for6 days in the absence of doxycycline, and were again stained withFITC-hFc for the presence of hFcγRI. Cells that did not stain for hFcγRIwere collected and expanded in culture medium containing 1 μg/ml ofdoxycycline for three days. The pool was then stained for the presenceof hFcγRI and were isolated by flow cytometry. Cells that expressed thehighest levels of hFcγRI (top 1%) were sorted onto 96 well plates at onecell per well. These cells presumably contained cell that had lownon-induced expression levels of FcγR1 and high inducible levels ofFcγR1. After expansion, the induction of hFcγRI by doxycycline in 20clones was confirmed by immunostaining with FITC-hFc and flow cytometry.One clone was chosen for further characterization and was named RGC10.

In the absence of doxycycline, RGC10 did not express detectable levelsof hFcγRI, whereas high levels of hFcγRI were observed in cells thatwere induced with 1 μg/ml of doxycycline for three days. The meanfluorescence of RGC10 cells increased by more than 1,000 fold afterinduction by doxycycline.

Example 6 Isolation of 4SC622-Producing Cell Lines from RGC10

RGC10 cells were transfected with pEE14.1-622, and MSX-resistant cellswere pooled after selection with 25 mM MSX for two weeks. Expression ofhFcγRI was induced by the addition of 1 μg/ml of doxycycline to theculture medium for three days. One mg/ml rat IgG was added to theculture medium containing doxycycline 18 hours prior to staining withpolyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment andanalysis by flow cytometry. Cells that expressed the highest levels of4SC622 (top 1%) were sorted into 96 well plates at 1 cell per well.Without induction of hFcγRI expression by doxycycline, staining withpolyclonal FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment failsto detect cell surface bound 4SC622. Sixty clones were expanded in theabsence of doxycycline. The specific productivity of the 13 highestproducers was determined by Pandex assay. The specific productivity ofclone 1C2 was 17.8 pg/cell/day, significantly better than the 12pg/cell/day observed for the best 4SC622 cell line previously isolatedusing the unregulated hFcγRI cell line RGC1.

Example 7 Sp2/0 Myeloma Cells can be Engineered to Express a CellSurface Capture Protein

In this example, the Sp2/0-Ag14 myeloma cell line was engineered tostably express hFcγRI to demonstrate that the autologous secretion trapmethod was applicable to cell lines other than CHO. The gene for hFcγRIwas introduced into the myeloma cell by retroviral infection. Theplasmid pLXRN (Clontech; Palo Alto, Calif.), a retroviral DNA vectorwherein a gene of interest may be expressed from the upstream Moloneymurine sarcoma virus long terminal repeat (MoMuSV LTR) promoter, wasused to generate retrovirus encoding the hFcγRI gene. The 1,363 bp Xho Ifragment from pTE084, encoding the human FcγRI gene, was cloned into theXho I site of pLXRN. A plasmid in which hFcγRI cDNA expression wasdependent on the MoMuSV LTR was chosen and named pTE255.

Pantropic retrovirus for the expression of hFcγRI was generatedessentially following the manufacturer's guidelines. The packaging cellline GP-293, a HEK 293-based cell line that stably expresses the viralgag and pol proteins (Clontech; Palo Alto, Calif.), was co-transfectedwith 10 mg each of pVSV-G and pTE255. The plasmid pVSV-G allowsexpression of the viral envelope protein VSV-G that confers broad hostrange upon the infective particles.

Construction of Sp2-hFcγRI-4.

The pantropic hFcγRI retrovirus was used to infect 1×10⁷ Sp2/0-Ag14myeloma cells (American Type Culture Collection; Manassas, Va.) at amultiplicity of about 10 infective particles per cell. Three days afterinfection, cells were stained for 1 hour then washed twice with PBSprior to analysis by flow cytometry. Those cells expressing hFcγRI, asindicated by bound FITC-hFc, were collected as a pool by flow cytometry.The pool was expanded for 13 days then again stained with FITC-hFc andcells expressing hFcγRI were collected as a pool by flow cytometry.These sorted cells were cultured in 10% fetal bovine serum 90%Dulbecco's Modified Eagle's Medium (DMEM) with 4.5 g/l glucose and 4 mMglutamine for 3 weeks, stained with FITC-hFc, and the cells with meanfluorescence in the top 1% of the population were cloned by single cellsorting. After expansion, 24 clones were examined by flow cytometry forexpression of hFcγRI, as described above, and one clone, Sp2-hFcγRI-4,was chosen for additional characterization.

Isolation of Sp2-hFcγRI-4 Cells Expressing 4SC622 Protein.

Sp2-hFcγRI-4 cells (1×10⁷) were transfected with pTE209, a plasmid thatallows constitutive expression of 4SC622 from the CMV-MIE promoter andconfers resistance to hygromycin. The transfected cells were placed inmedium containing 10% FCS, 90% D-MEM and 400 μg/ml hygromycin for 14days. Hygromycin-resistant cells were incubated with 1 mg/ml rabbit IgGfor eighteen hours prior to staining with polyclonal FITC-conjugatedanti-human IgG (H+L) F (ab′)₂ fragment. Cells were stained for 1 hourthen washed twice with PBS prior to analysis by flow cytometry. Labeledcells were collected as a pool by flow cytometry then cultured for 5days and sorted as described above. Cells from the expanded pool thatbound the most polyclonal FITC-conjugated anti-human IgG (H+L) F (ab′)₂fragment, top 1% population, were then cloned by single cell sorting(FIG. 17). Production of 4SC622 from ten clones was analyzed by ELISAand all 10 clones were found to express 4SC622; clone 5H11 produced4SC622 at 0.5 pg per cell per day. These data showed that clonessecreting 4SC622 were efficiently isolated by the autologous secretiontrap method from a heterogeneous pool of cells derived from stabletransfection of Sp2-hFcγRI-4 cells with pTE209.

To confirm that 4SC622 was autologously displayed on the surface ofmyeloma cells expressing both 4SC622 and hFcγRI, clone 5H11 wasincubated with 1 mg/ml rabbit IgG for 18 hours then stained withFITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment and found todisplay cell surface 4SC622. Secreted protein was displayed underconditions in which cross-feeding was blocked by rabbit IgG,demonstrating the autologous display of 4SC622. These data indicatedthat the autologous secretion trap method described above was notlimited to CHO cells and may be extended to myeloma and other cell typesas well.

Example 8 Protein G Chimeric Protein can Function as a Cell SurfaceCapture Protein

To demonstrate the application of the autologous secretion trap methodto a cell surface capture protein other than hFcγRI, a cell lineexpressing Protein G was constructed. Protein G, from the Streptococcusstrain G148, binds to all human and mouse IgG subclasses, and as suchhas utility for the isolation of recombinant cells expressing antibodiesor IgG Fc fusion proteins. To demonstrate that the Protein G IgG Fcbinding domain could be used as a cell surface capture protein capableof binding to all human and mouse IgG subclasses, we constructed a CHOline expressing a chimeric protein comprised of the Fc binding domain ofProtein G fused to the hFcγRI transmembrane and intracellular domain.The Fc binding domain of Protein G contains three homologous repeats of55 amino acids long (Guss et al., (1986) EMBO 5:1567 and Sjobring etal., (1991) J. Biol. Chem. 266:399) and each repeat is capable ofbinding one IgG Fc. To improve the expression of this chimeric proteinin CHO cells, we constructed a synthetic DNA in which the signalsequence from the mouse RORI gene was fused to the Fc binding domain,amino acids 303 to 497 of Protein G (accession # X06173) (SEQ ID NO:1).This synthetic DNA was generated by a combination of oligonucleotideannealing, gap filling, and PCR amplification. The synthetic DNA wasthen fused, by PCR, to DNA encoding the transmembrane and intracellulardomains, amino acids 279 to 374 (SEQ ID NO:2), of hFcγRI (accessionM21091). The resultant DNA encoding the Protein G/hFcγRI chimericprotein was cloned into pTE158 downstream of the CMV-MIE promoter,replacing the gene encoding hFcγRI, to yield the plasmid pTE300.

A CHO K1 cell line adapted to grow in serum-free medium, RGC14, wastransfected with pTE300, and after three days 400 μg/ml G418 was addedto the culture medium to select for stable integration of pTE300. Twoweeks after the start of selection, the cells were stained with FITC-hFcto identify cells that expressed hFcγRI. These cells were analyzed byflow cytometry and cells expressing hFcγRI were collected as a pool. Thecells were expanded for 10 days and the population of cells expressinghFcγRI was again isolated by flow cytometry. The cells were againexpanded, stained with FITC-hFc, and single cells expressing high levelsof the Protein G/hFcγRI chimeric protein were isolated by flowcytometry. Single cells that stained positive for FITC-hFc binding weresorted into medium composed of 10% fetal bovine serum, 90% Ham's F12,and 400 μg/ml G418. After two weeks incubation, 48 clones were examinedfor binding to bovine IgG present in the culture medium by staining withFITC-conjugated anti-bovine IgG F(ab′)₂ fragment (Jackson ImmunoResearchLaboratories, West Grove, Pa.). One clone, RGC18 that stained positivewith this antibody was chosen for further characterization.

Isolation of expression clones in RGC18: RGC18 cells (6×10⁶) weretransfected with pTE209 and selected for integration of the plasmid bygrowth in 400 μg/ml hygromycin for 18 days. Hygromycin-resistant cellswere incubated with 1 mg/ml rabbit IgG for eighteen hours prior tostaining with polyclonal FITC-conjugated anti-human IgG (H+L) F (ab′)₂fragment. Cells were stained for 1 hour then washed twice with PBS priorto analysis by flow cytometry. The most fluorescent cells (top 5%) wereisolated by single cell sorting and expanded for 3 weeks. Ten cloneswere examined for 4SC622 secretion. All clones tested secreted 4SC622 athigh level, and the best clone, RGC19, had a specific productivity of6.4 pg/cell day. This result demonstrated that 4SC622-expressing cellswere efficiently isolated from a heterogeneous pool of cells derivedfrom stable transfection of RGC18 with pTE209 by the autologoussecretion trap method. Furthermore, these data clearly demonstrated thata fragment of Protein G could be engineered to include a signal sequenceand transmembrane domain, and function as a cell surface captureprotein.

To confirm that 4SC622 was autologously displayed on the surface ofRGC19 cells expressing both Protein G/hFcγRI chimeric protein and4SC622, RGC19 was incubated with 1 mg/ml rabbit IgG for 18 hours thenstained with FITC-conjugated anti-human IgG (H+L) F(ab′)₂ fragment andanalyzed by flow cytometry. RGC19 cells were found to possess cellsurface 4SC622 under these conditions in which cross-feeding was blockedby rabbit IgG, suggesting autologous display of 4SC622. Rabbit IgGeffectively blocked binding of exogenous 4SC622 protein to RGC18 cells,but did not block display of 4SC622 on the cell surface of cellsexpressing 4SC622. These data demonstrated that the properties of theProtein G/hFcγRI chimeric protein were similar to those of hFcγRI as acell surface capture protein, and suggested that the autologoussecretion trap method can employ other proteins as cell surface captureproteins.

Example 9 Isolation of Antibody-Producing Cells from RGC10

To demonstrate the utility of the autologous secretion trap method forthe isolation of CHO cell lines that express recombinant antibodies wecloned the DNA encoding variable light and variable heavy genes from theKD5 hybridoma. KD5 is a hybridoma that expresses a monoclonal antibodyspecific for the human Tie-2 receptor.

The mouse IgG constant region gene sequences were cloned from 500 ng ofmouse spleen polyA+ RNA (Clontech, Palo Alto, Calif.). Single strandedcDNA was synthesized using SuperScript First-Strand Synthesis System forRT-PCR, primed with 50 ng of random hexamers (Invitrogen LifeTechnologies, Carlsbad, Calif.). The mouse kappa light constant DNAsequence (accession # Z37499) was amplified from this cDNA by PCR usingthe primers 5′ mCLK1 (Z37499) (5′-CGGGCTGATG CTGCACCAAC TGTATCCATCTTC-3′) (SEQ ID NO:3) and 3′ mCLK1 (Z37499) (5′-ACACTCTCCC CTGTTGAAGCTCTTGACAAT GGG-3′) (SEQ ID NO:4). The mouse IgG2a constant region DNAsequence (accession # AJ294738) was also amplified from this cDNA by PCRusing the primers 5′ mCH2a (AJ294738) (5′-GCCAAAACAA CAGCCCCATCGGTCTATCCA C-3′) (SEQ ID NO:5) and 3′ mCH2a (AJ294738) (5′-TCATTTACCCGGAGTCCGGG AGAAGCTCTT AGTCG-3′) (SEQ ID NO:6). The PCR products werecloned into pCR2.1-TOPO using TOPO TA Cloning kit (Invitrogen LifeTechnologies, Carlsbad, Calif.) and the sequence of the constant regionswere verified.

The KD5 variable region genes were amplified by RT-PCR from KD5hybridoma mRNA and cloned into pCR2.1-TOPO using the heavy and lightchain variable region primer mixes from Amersham-Pharmacia Biotech(Piscataway, N.J.). The variable heavy chain gene was PCR amplifiedusing the pCR2.1-TOPO cloned variable region as template with theprimers 5′ BspMI/KD5VH N-term (5′-GAGAGTACCT GCGTCATGCA GATGTGAAACTGCAGGAGTC TGGCCCT-3′) (SEQ ID NO:7) and 3′ BspMI/KD5VH C-term(5′-GAGAGACCTG CGTCAGCTGA GGAGACGGTG ACCGTGGT-3′) (SEQ ID NO:8),digested with BspMI and ligated to the BsaI-digested IgG2a constantheavy gene PCR fragment amplified with the primers 5′ BsaI/CH2a N-term(5′-GAGAGGGTCT CACAGCCAAA ACAACAGCCC CATCG-3′) (SEQ ID NO:9) and 3′BsaI/CH2a C-term (5′-GAGAGGGTCT CCGGCCGCTC ATTTACCCGG AGTCCGGG AGAA-3′)(SEQ ID NO:10). This fragment was then ligated into the BspMI and NotIsites of pRG882. The resulting plasmid, pTE317, was capable ofexpressing the KD5 recombinant heavy chain gene, fused to the mROR1signal sequence, from the CMV-MIE promoter. The variable light chaingene was PCR amplified using the pCR2.1-TOPO cloned variable region astemplate with the primers 5′ BsmBI/KD5VL N-term (5′-GAGAGCGTCTCATGCAGACA TCCAGATGAC CCAGTCTCCA-3′) (SEQ ID NO:11) and 3′ BsmBI/KD5VLC-term (5′-GAGAGCGTCT CACAGCCCGT TTTATTTCCA GCTTGGTCCC-3′) (SEQ IDNO:12), digested with BsmBI and ligated to the BsaI-digested kappaconstant light gene PCR fragment amplified with the primers 5′ BsaI/CLKN-term (5′-GAGAGGGTCT CAGCTGATGC TGCACCAACT GTATCC-3′) (SEQ ID NO:13)and 3′ BsaI/CLK C-term (5′-GAGAGGGTCT CAGGCCGCTC AACACTCTCC CCTGTTGAAGCTCTTGAC-3′) (SEQ ID NO:14). This fragment was then ligated into theBspMI and NotI sites of pRG882. The resulting plasmid, pTE316, wascapable of expressing the KD5 recombinant light chain gene, fused to themROR1 signal sequence, from the CMV-MIE promoter.

The 1450 bp EcoRI-NotI fragment from pTE317, encoding the KD5 heavychain gene, was cloned into the EcoRI and NotI sites of pRG980, a vectorthat confers resistance to hygromycin and allows expression ofrecombinant genes for the UbC promoter, to yield plasmid pTE322.Similarly, the 750 bp EcoRI-NotI fragment from pTE316, encoding the KD5light chain gene, was cloned into the EcoRI and NotI sites of pRG985, avector that confers resistance to puromycin and allows expression ofrecombinant genes for the UbC promoter, to yield plasmid pTE324. RGC10cells (5×10⁶) were transfected with 3 μg pTE322 and 3 μg pTE322 andselected for integration of the plasmids by growth in F12 mediumsupplemented with 10% fetal calf serum with 20 μg puromycin and 400μg/ml hygromycin for 14 days. Expression of hFcγRI was induced by theaddition of 1 μg/ml of doxycycline to the culture medium for three days.Double-resistant cells were incubated with 1 mg/ml rabbit IgG foreighteen hours prior to staining with goat polyclonal FITC-conjugatedanti-mouse IgG (Fcγ) F (ab′)₂ fragment (Jackson ImmunoResearchLaboratories, West Grove, Pa.). Cells were stained for 1 hour thenwashed twice with PBS prior to analysis by flow cytometry. The mostfluorescent cells (top 5%) were isolated as a pool and expanded for 10days, after which the protocol was repeated but the top 1% mostfluorescent cells were isolated as a pool. This pool was expanded for 10days then the top 0.1% most fluorescent cells were isolated as singlecells into 96-well plates. Clones were analyzed by ELISA for expressionof antibody and seven clones were chosen from 53 clones analyzed. Theaverage specific productivity of these clones was 35 pg/cell/day and thebest clone expressed the recombinant KD5 monoclonal antibody at 54pg/cell/day.

Example 10 FASTR Screens Unaffected by CSCP Expression Level

To demonstrate that the expression level of the CSCP does notsignificantly affect the ability to isolate cells expressing anassociated sPOI, FASTR screens for the same sPOI in two different hostcell lines that each express the same CSCP but at either a high level ora low level were compared.

The FASTR host cell line RGC10 was selected for high-level expression ofhFcγRI protein by stable integration of pTE158 and was found to contain40 hFcγRI integrated gene copies. A new cell line, RS527, that expressedhFcγRI protein at a lower level, was generated from CHO K1 after stabletransfection and selection for single copy gene integration. RS527 cellsexpressed significantly less hFcγRI protein than RGC10 cells asdetermined by Western blot analysis of whole cell lysates of the FASTRcell lines.

Briefly, RGC10 and RS527 cells were transfected with pTE462, a plasmidcapable of expressing a secreted hFc-fusion protein Rc1-hFc andconferring resistance to hygromycin. The transfected cultures wereselected with hygromycin for two weeks. The hygromycin-resistant cellswere induced with 1 μg/ml doxycycline (Dox) and blocked with rabbit IgGovernight, following the FASTR method described herein. The next day,the RGC10/pTE462 and RS527/pTE462 cultures were stained by aFITC-conjugated antibody specific for hFc and then analyzed by flowcytometry. Three cell bins R4, R5, and R6 marking cells with low,medium, and high fluorescence respectively were sorted from each hostline and expanded in tissue culture.

To compare Rc1-hFc protein production level from the six cell bins, sixcultures were set up using equal number of cells for each bin. Threedays later, conditioned media were collected. The Rc1-hFc protein titersin the conditioned media were determined by ELISA and were plottedagainst mean fluorescence of the respective cell bins. For both RGC10and RS527 host lines, there was a similar correlation between meanfluorescence (amount of Rc1-hFc displayed on the cell surface) and sPOIprotein production levels of the isolated cell pools. Mostsignificantly, the sPOI titers in the two high fluorescence R6 binsderived from RGC10 and RS527 were similar. These data demonstrate thatthe expression level of the CSCP in a FASTR host cell line does notsignificantly affect the use of that host to isolate transfected cellsbased on expression level of a sPOI.

Example 11 Tie2 Receptor as a Cell Surface Capture Protein

Cell surface capture proteins (CSCP's) other than FcγRI can be used inthe methods described herein. In this example, the Tie2 receptorfunctions as a CSCP and is used to isolate cells expressing aTie-specific ScFv_(C1b)-Fc fusion protein made from the C1b monoclonalantibody that specifically binds the extracellular domain of Tie2receptor. Although the CSCP for ScFv_(C1b)-Fc can be hFcγRI, thisexample demonstrates that Tie2 can also be used as the CSCP forScFv_(C1b)-Fc.

To construct an inducible Tie2 CSCP cell line, CHO K1 was first stablytransfected with the TetR plasmid pcDNA6/TR. The blasticidin-resistantcell pool was then stably transfected with pTE259, a plasmid that allowsinducible expression of a protein comprised of the extracellular domainand transmembrane domain of Tie2. Inducible cell clones were isolated byflow cytometry after staining with an antibody specific for Tie2 (FIG.3). The RGC54 clone was chosen to study the feasibility of FASTR for theexpression of ScFv_(C1b)-Fc.

RGC54 cells were stably transfected with pTE988, a plasmid capable ofexpressing the secreted hFc-fusion protein ScFv_(C1b)-Fc and conferringresistance to hygromycin. The transfected culture was selected withhygromycin for two weeks. The hygromycin-resistant cells were inducedwith Dox and blocked with 1 mg/ml of purified C1b mAb. The C1bmonoclonal antibody was the source of the variable regions inScFv_(C1b)-Fc. The next day, the cell pool was stained by aFITC-conjugated antibody specific for hFc and then analyzed by flowcytometry. Three cell bins R6, R7, and R8 marking cells with high,medium, and low fluorescence respectively were sorted and expanded intissue culture. Three cultures were set up using an equal number ofcells for each bin to determine ScFv_(C1b)-Fc protein production asdetermined by ELISA. A correlation existed between mean fluorescence(amount of ScFv_(C1b)-Fc binding to Tie2 on the cell surface) andScFv_(C1b)-Fc protein production levels of the isolated cell pools.

These data show that CSCP other than hFcγRI can serve as a CSCP, andalso suggest that any receptor may be converted into a CSCP by removalof its cytoplasmic domain. These data also demonstrate that an antigencan be made into a CSCP and used for FASTR screening cells expressing anantigen-specific antibody-related molecule.

Example 12 Effective FASTR Screens with CSCP:sPOI Pairs Having LowAffinity

Angiopoetin-1 is a ligand for the Tie2 receptor. A chimeric proteincomprising angiopoetin-1 receptor binding domain and hFc (FD1-hFc) bindsto Tie2 with an affinity constant of 174 nM as determined by BIAcore™.FD1-hFc and Tie2 were chosen as sPOI and CSCP, respectively, todetermine if a minimum affinity between CSCP and sPOI is required forFASTR screens

In cell decoration experiments, exogenously added FD1-hFc boundspecifically to RGC54 cells through Tie2. To determine if the affinitybetween Tie2 and FD1-hFc is sufficient to allow FASTR screening, RGC54cells were stably transfected with pTE942, a plasmid capable ofexpressing the secreted hFc-fusion protein FD1-hFc and conferringresistance to hygromycin. The transfected culture was selected withhygromycin for two weeks. The hygromycin-resistant cells were inducedwith Dox and blocked with 1 mg/ml of purified FD1-mFc comprising mouseIgG1 Fc. The next day, the cell pool was stained by a FITC-conjugatedantibody specific for hFc and then analyzed by flow cytometry. Threecell bins R6, R7, and R8 marking cells with high, medium, and lowfluorescence, respectively, were collected. Cultures were set up usingequal number of cells for each bin to determine FD1-hFc proteinproduction levels in the conditioned media as determined by ELISA. Therewas a correlation between mean fluorescence (FD1-Fc binding to cellsurface-bound Tie2) and FD1-hFc protein production levels of theisolated cell pools. The bin with the highest fluorescence produced themost FD1-hFc.

These data demonstrate that a CSCP:sPOI pair with low affinity (174 nMKD) can be used for effective FASTR screens. Importantly, thedissociation t_(1/2) for FD1-Fc: Tie2 binding is less than 2 minutes,suggesting that any CSCP:sPOI pair with a measurable affinity can workin FASTR screens. In addition, this experiment also shows that anon-FcγRI receptor may be used as the CSCP to isolate cells expressingits ligand.

Example 12 Fusing a Transmembrane Domain onto an ScFv Makes a FunctionalCSCP

An CSCP can be any cell surface-bound protein that has a measurableaffinity to the sPOI. To demonstrate this, a totally synthetic CSCP wasconstructed by fusing the transmembrane domain from the PDGF receptor toan ScFv containing the variable regions from the murine kappachain-specific monoclonal antibody HB58. A FASTR host was constructedthat expresses this chimeric protein (ScFv_(HB58)-TM_(PDGFR)) and wasused to isolate cells expressing the angiopoeitin-2 FD domain-specificP12 antibody.

The RS655 cell line, derived from CHO K1, constitutively expressesSCFV_(HB58)-TM_(PDGFR). Cells expressing ScFv_(HB58)-TM_(PDGFR) can bestained by sequential incubation with P12 mAb, FD2-hFc, andFITC-conjugated anti-hIgG—P12 captured on the cell surface by the HB58ScFv was detected by its affinity for FD2, which in turn was detected byrecognition of the hFc tag. RS656 cells were derived from RS655 cellsafter stable transfection with a plasmid encoding the gene for eYFP.Nearly 100% of RS656 cells were eYFP-positive, and most (76%) maintainedexpression of ScFv_(HB58)-TM_(PDGFR) as detected by binding to FD2-hFc.

RS655 cells were stably transfected with pTE693, a plasmid capable ofexpressing the heavy and light chains of the P12 antibody, andconferring resistance to puromycin. The transfected culture was selectedwith puromycin for two weeks to yield a pool of cells that wereheterogeneous with regard to P12 mAb expression (RS655/pTE693).

To determine if ScFv_(HB58)-TM_(PDGFR) could function as a CSCP andfacilitate isolation of antibody-producing cells from non-producers,equal numbers of RS656 cells and RS655/pTE693 cells were mixed andco-cultured. When P12 expressed from RS655/pTE693 cells was allowed todiffuse and bind to ScFv_(HB58) on the surface of RS656 cells a largepopulation of yellow cells were also positive for binding FD2-hFc.However, if the ScFv_(HB58) on the surface of RS656 was bound withexcess murine IgG, then only non-yellow cells were positive for bindingFD2-hFc, demonstrating that expressing cells were effectively separatedfrom non-expressing cells.

These data demonstrate that an ScFv can be made into a functional CSCPby targeting it to the cell membrane. The data also show that FASTRallows cells expressing a secreted antibody to be detected with theantibody's antigen.

Example 13 A Protein of Interest Comprising a T Cell Receptor VariableRegion

A flow cytometry-based autologous secretion trap (FASTR) method forisolating high expression clones of a cell line that expresses a proteinof interest that is a TCR-Fc is prepared in a manner analogous topreparing a cell line that expresses an antibody of interest. Highexpression clones are identified by screening cells that display ontheir surface the TCR-Fc of interest bound to hFcγR.

In these examples, the CHO K1 cell line RGC10, comprising an inducibleFcγR1 as a cell surface capture molecule, is employed. RGC10 is made toexpress recombinant TCR-Fc's by cloning TCR variable regions, in frame,to a human Fc region either directly in frame or with a linker sequencebetween the TCR variable regions and the human Fc region.

To make a protein of interest that is a dimer comprising an Fc-linkedTCR α variable domain and an Fc-linked TCR β variable domain, RGC10 istransfected with two vectors: a first vector capable of expressing a TCRα variable domain fusion protein with a human Fc sequence, and a secondvector capable of expressing a TCR β domain fusion protein with the samehuman Fc sequence. Each vector includes leader sequence (e.g., asecretion signal sequence) 5′ with respect to the TCR variable region.and a selectable marker that is a drug resistance gene. Following eachvector transfection, cells containing the vector are selected by anappropriate drug selection. The selection results in an RGC10 cell linehaving both the first and the second vectors. Cells expressing proteinsof interest can be detected by one or more of an antibody to the βvariable domain, an antibody to the α variable domain, and an antibodyto the Fc domain.

To make a protein of interest that is a dimer comprising both an α and aβ TCR variable domain fused to an Fc, RGC10 is transfected with a singlevector encoding a protein of interest that is constructed as follows: aleader sequence (e.g., a secretion signal sequence), followed by a TCRvariable β domain fused to a linker, where the linker is, in turn, fusedto a TCR variable α domain, which in turn is fused to an Fc sequence.Alternatively, the single vector can be constructed as follows: a leadersequence (e.g., a secretion signal sequence), followed by a TCR variableα domain fused to a linker, where the linker is, in turn, fused to a TCRvariable β domain, which in turn is fused to an Fc sequence. Cellsexpressing proteins of interest can be detected by one or more of anantibody to the β variable domain, an antibody to the α variable domain,and an antibody to the Fc domain.

To make proteins of interest, as above, which also comprise a TCR αand/or TCR β constant domain, the TCR variable domain (α or β) is fusedto a TCR constant domain (e.g., TCR variable domain α is fused to TCRconstant domain α, and TCR variable domain β is fused to TCR constantdomain β), and the TCR variable+constant domain is fused directly orthrough a linker to the Fc domain. Cells expressing proteins of interestcan be detected by one or more of an antibody to the β variable domain,an antibody to the α variable domain, and an antibody to the Fc domain.

Cells expressing desired amounts of the TCR-Fc are isolated using thesame procedure as used in isolating 4SC622-producing cell linesdescribed herein, using one or more of an antibody to the α variabledomain, an antibody to the β variable domain, an antibody to the αconstant domain, and antibody to the β constant domain, and an antibodyto the Fc domain. Cells expressing the highest levels of the TCR-Fc areselected as TCR-Fc-producing cell lines.

Although the foregoing invention has been described in some detail byway of illustration and example, it will be readily apparent to those ofordinary skill in the art that certain changes and modifications may bemade to the teachings of the invention without departing from the spiritor scope of the appended claims.

21. A high expression recombinant cell line comprising: (a) a cellsurface capture molecule encoded by a first stably integrated vector;and (b) a secretable protein of interest (POI) encoded by a secondstably integrated vector; wherein the POI and cell surface capturemolecule are capable of binding to each other and forming a capturemolecule: POI (CM:POI) complex, and wherein the CM:POI complex is formedintracellularly and displayed on the cell surface.
 22. The cell line ofclaim 21, wherein the cell surface capture molecule comprises an Fcreceptor or a fragment thereof, and the POI comprises a fusion proteinconsisting of the Fc domain of IgG (Fc-fusion protein).
 23. The cellline of claim 22, wherein the Fc receptor comprises hFcγR1, or afragment thereof.
 24. The cell line of claim 22, wherein the IgG ishuman IgG1.
 25. The cell line of claim 22, wherein the Fc-fusion proteinis a chimeric molecule comprising a receptor extracellular domain fusedto an IgG Fc domain.
 26. The cell line of claim 25, wherein theFc-fusion protein is selected from the group consisting ofIL-2Rγ-hIL4Rγ-Fc, hVEGFR1R2-Fc, hVEGFR1-R3-Fc, and Angiopoietin-1receptor binding domain-Fc.
 27. The cell line of claim 21, wherein thecell surface capture molecule comprises a ligand-specific receptor or afragment thereof, and the POI comprises a receptor ligand, or a fragmentthereof.
 28. The cell line of claim 27, wherein the ligand-specificreceptor is Tie2 receptor or a fragment thereof, and the receptor ligandis angiopoietin-1 or a fragment thereof.
 29. The cell line of claim 21,wherein the cell surface capture molecule comprises a receptor-specificligand or a fragment thereof, and the POI comprises a soluble receptorprotein, or a fragment thereof.
 30. The cell line of claim 21, whereinthe cell surface capture molecule is selected form the group consistingof an Fc receptor, a protein with an Fc binding domain, and a proteinfused to an Fc binding domain, and the POI is selected from the groupconsisting of an antibody, a single chain antibody (ScFv), and a fusionprotein comprising the Fc domain of IgG.
 31. The cell line of claim 30,wherein the Fc binding domain is selected from the group consisting ofProtein A, Protein G, and Protein H, or a functional fragment thereof.32. The cell line of claim 30, wherein the Fc binding domain comprisesamino acids 303 to 497 of Protein G (SEQ ID NO:1).
 33. The cell line ofclaim 21, wherein the first stably integrated vector comprises DNAencoding a cell surface capture molecule (CSCM) downstream of a promoterand the CSCM is under transcriptional control of such promoter, whereinthe CSCM is capable of binding the secretable POI under conditions inwhich such CSCM is expressed.
 34. The cell line of claim 21, wherein thesecond stably integrated vector comprises DNA encoding a POI downstreamof a promoter and the POI is under transcriptional control of suchpromoter.
 35. The vector of claim 33, wherein the vector is a plasmid.36. The vector of claim 34, wherein the vector is a plasmid.
 37. Thevector of claim 33 comprising a CMV-MIE/TetO hybrid promoter.
 38. Thevector of claim 33 comprising DNA encoding a CSCM selected from thegroup consisting of a ligand-specific receptor, a receptor-specificligand, an Fc receptor, a protein with an Fc binding domain, and aprotein fused to an Fc binding domain, or a fragment thereof.
 39. Thevector of claim 33 comprising DNA encoding the Fc binding domain ofProtein G comprising amino acids 303 to 497 of Protein G (SEQ ID NO:1).40. The vector of claim 34 comprising DNA encoding a POI selected fromthe group consisting of a growth factor, an antibody, a single chainantibody (ScFv), a fusion protein comprising the Fc domain of IgG(Fc-fusion protein), a soluble receptor protein, and a receptor ligand,or a fragment thereof.
 41. The vector of claim 34 comprising DNAencoding a POI selected from the group consisting of Interleukin (IL)-1,IL-2, IL-4, IL-5, IL-6, IL-7, IL-9, IL-10, IL-13, IL-15, IL-16, IL-17,IL-18, IL-21, Ciliary Neurotrophic Factor (CNTF), erythropoietin,Vascular Endothelial Growth Factor (VEGF), angiopoietin 1 (Ang-1),angiopoietin 2 (Ang-2), TNF, Interferon-gamma, GM-CSF, and TGFβ.
 42. Thevector of claim 40 comprising DNA encoding a receptor extracellulardomain fused to an Fc domain of IgG.
 43. The cell line of claim 33,wherein the first stably integrated vector comprises a CSCM undertranscriptional control of a promoter whose activity is regulated byTetR, and the cell line further comprises a stably integrated vectorcapable of expressing the tetracycline repressor protein (TetR).
 44. Thecell line of claim 43, wherein the first stably integrated vectorcomprising a CSCM under transcriptional control of a CMV-MIE/TetO hybridpromoter.
 45. The cell line of claim 44, wherein expression of the CSCMis induced by tetracycline, or the analog doxycycline.
 46. The cell lineof claim 43, wherein the promoter of the first stably integrated vectorcontains two tandem TetR operators (2× TetO).
 47. The cell line of claim22, made by the process of: (a) transfecting a host cell with (i) afirst vector comprising DNA encoding the Fc receptor or fragmentthereof, and (ii) a second vector comprising DNA encoding the Fc-fusionprotein, wherein the Fc-fusion protein is capable of being secreted; (b)allowing the cells to stably express both the Fc receptor and Fc-fusionprotein and bind to form a Fc receptor:Fc-fusion protein complex; (c)contacting cells having such Fc receptor:Fc-fusion protein complexdisplayed on the cell-surface with a detection molecule that binds theFc-fusion protein, wherein said contacting is carried out in thepresence of a blocking molecule that binds the Fc receptor and preventsbinding of said Fc-fusion protein to said Fc receptor but does not bindto said detection molecule; (d) detecting cells containing Fc-fusionprotein bound to said detection molecule; (e) isolating a cell based onthe detection molecule; and (f) culturing the cell line.
 48. A method ofmaking the cell line of claim 22, comprising (a) transfecting a hostcell with (i) a first vector comprising DNA encoding the Fc receptor orfragment thereof, and (ii) a second vector comprising DNA encoding theFc-fusion protein, wherein the Fc-fusion protein is capable of beingsecreted; (b) allowing the cells to stably express both the Fc receptorand Fc-fusion protein and bind to form a Fc receptor:Fc-fusion proteincomplex; (c) contacting cells having a cell surface-displayed Fcreceptor:Fc-fusion protein complex with a detection molecule that bindsthe Fc-fusion protein, wherein said contacting is carried out in thepresence of a blocking molecule that binds the Fc receptor and preventsbinding of said Fc-fusion protein to said Fc receptor but does not bindto said detection molecule; (d) detecting cells containing Fc-fusionprotein bound to said detection molecule; (e) isolating a cell based onthe detection molecule; and (f) culturing the cell line.
 49. The methodof claim 48, wherein the cell is isolated by flow cytometry.
 50. Themethod of claim 48, wherein the detection molecule is bound to a solidsupport or particle.
 51. The method of claim 48, wherein the Fc-fusionprotein is detected at step (c) by contacting said cell with twodetection molecules that bind each other and are differentially labeled.52. The method of claim 48, wherein the blocking molecule that binds theFc receptor is added to the cell at step (c) prior to detecting theFc-fusion protein to reduce the diffusion and binding of the Fc-fusionprotein to a neighboring cell.
 53. The method of claim 49, performed ona population of cells, wherein the isolating step isolates the cellsbinding to the detection molecule from the population.
 54. The method ofclaim 53, wherein the cells in a population of cells express differentlevels of the Fc-fusion protein, and the isolation step isolates a cellbased on relative expression level of Fc-fusion protein.